The present invention is concerned with a process for the purification of polyolefins and, more particularly, with a process for removing catalyst residues present in polyolefins obtained by Ziegler-Natta catalysis and with the purified polyolefins having a very low residual content of impurities thus obtained.
In numerous processes for the synthesis of polymers of ethylene, propylene, 1-butene or 1-hexene, the reaction product contains metallic residues resulting from the use of organo-metallic catalysts, such as alkyl-aluminums and titanium chlorides. Regardless of the catalysts employed, the polymeric product isolated from the reaction mixture exhibits several short-comings due to the presence of catalyst residues: it readily becomes discolored and becomes unstable after a long period of storage or when heated. Although methods of overcoming these disadvantages are known, the most economical method of avoiding them is to remove the catalyst residues at the time of production of the polymer.
In processes in which an olefin is polymerized by means of a Ziegler-Natta catalyst in suspension in a diluent, several means for reducing the amount of catalyst residues in the polymer have been described in the prior art. For example, British Specification No. 887,315 describes a process in which the polymer is reacted, at a temperature above 70.degree. C., with a heavy alcohol in the presence of a liquid hydrocarbon and an acid chloride, and the polymer is washed by means of a liquid hydrocarbon. This reaction is not very efficient since, as stated in example 3 of the British specification, the amount of catalyst residues is only reduced to 300 ppm when 48 mols of acetyl chloride per mol of Ziegler-Natta catalyst are used. More efficient processes are also known in which, still in the presence of an alcohol, either a reactant that is hazardous to handle such as a mineral acid (British Specification No. 1,080,140) or an expensive reactant such as an organic ester is used. The content of each catalytic metal (aluminum, titanium) may not be reduced under 10 ppm in commercial homopolymers by means of these processes.
It is thus an object of the invention to provide a process for the purification of polyolefins using reactants which do not give rise to the aforementioned problems.
Another object of the invention is to provide such a process which provides polyolefins with a content of catalyst residues reduced to an amount at least as low as that of known commercial products.